chore(sonar): adopt Has.Count, Is.Empty, Does.Contain, char overloads in tests (NUnit2046, NUnit2011, CA1866)

- NUnit2046: `Assert.That(x.Count, Is.EqualTo(n))` → `Assert.That(x, Has.Count.EqualTo(n))` (or `Is.Empty` when n==0)
- NUnit2011: `Assert.That(s.Contains(x))` → `Assert.That(s, Does.Contain(x))` for richer failure messages
- CA1866: `.StartsWith("$"|"@"|":")` → `.StartsWith('$'|'@'|':')` char overload

Driven by `dotnet format analyzers --diagnostics NUnit2046 NUnit2011 CA1866`
for the cases the Roslyn fixer handles, plus a regex sweep for the remaining
`Count == n` (n>0) cases which the fixer doesn't address. CA1866 had no
associated code fix and was edited by hand (3 sites in 2 files). All tests
green (1180 passing).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Strata.SqlTools.SqlBreakdown.Tests/Extensions/QueryBreakdownExtensionsTests.cs

@@ -188,7 +188,7 @@ public class QueryBreakdownExtensionsTests
 
         // Assert
         Assert.That(query.IsUsingWithClause, Is.True);
-        Assert.That(query.WithClauses.Count, Is.EqualTo(1));
+        Assert.That(query.WithClauses, Has.Count.EqualTo(1));
         Assert.That(query.WithClauses[0].TableName, Is.EqualTo("active_users"));
         Assert.That(query.SelectClause.Clause, Is.EqualTo("*"));
         Assert.That(query.FromClause.Clause, Is.EqualTo("active_users"));
@@ -238,7 +238,7 @@ public class QueryBreakdownExtensionsTests
 
         // Assert
         Assert.That(query.IsUsingWithClause, Is.True);
-        Assert.That(query.WithClauses.Count, Is.EqualTo(2));
+        Assert.That(query.WithClauses, Has.Count.EqualTo(2));
         Assert.That(query.WithClauses[0].TableName, Is.EqualTo("active_users"));
         Assert.That(query.WithClauses[1].TableName, Is.EqualTo("recent_orders"));
     }
@@ -261,7 +261,7 @@ public class QueryBreakdownExtensionsTests
 
         // Assert
         Assert.That(query.IsUsingWithClause, Is.True);
-        Assert.That(query.WithClauses.Count, Is.EqualTo(1));
+        Assert.That(query.WithClauses, Has.Count.EqualTo(1));
         Assert.That(sql, Does.Contain("active_users (id, name, email)"));
     }
 
@@ -282,7 +282,7 @@ public class QueryBreakdownExtensionsTests
 
         // Assert
         Assert.That(query.IsUsingWithClause, Is.True);
-        Assert.That(query.WithClauses.Count, Is.EqualTo(1));
+        Assert.That(query.WithClauses, Has.Count.EqualTo(1));
         Assert.That(query.WithClauses[0].TableName, Is.EqualTo("active_users"));
     }
 
@@ -356,7 +356,7 @@ public class QueryBreakdownExtensionsTests
         var sql = query.GetSql();
 
         // Assert
-        Assert.That(query.WithClauses.Count, Is.EqualTo(2));
+        Assert.That(query.WithClauses, Has.Count.EqualTo(2));
         Assert.That(sql, Does.Contain("active_users AS"));
         Assert.That(sql, Does.Contain("user_orders AS"));
         Assert.That(sql, Does.Contain("customer_tier"));